1. Field of the Invention
The present invention relates to piezoelectric filters of the trapped energy type, which may have pass bands of varying widths and improved symmetry about a center frequency.
2. Description of the Prior Act
The prior art will be described with reference to FIGS. 1-3 of the accompanying drawings. FIG. 1 is a top or plan view of a piezoelectric filter of the prior art, FIG. 2 is a sectional side view thereof, and FIG. 3 shows a characteristic curve for such a prior art filter.
Generally, a piezoelectric filer of the trapped energy type comprises a piezoelectric wafer 1 made of, for example, a polarized ceramic material, input and output electrodes 3, 5, and a ground electrode 7. The input and output electrodes are secured to one major surface of the piezoelectric wafer with their adjacent edges spaced apart by a continuous, constant width gap, and the ground electrode is secured to the other major surface of the wafer opposite to and underlying the input and output electrodes. The configuration of the ground electrode is substantially the same as the combined configuration of the input and output electrodes. This type of filter makes use of the fact that the oscillating energy within the wafer is generally confined to the portion thereof where the input and output electrodes and the ground electrode face each other and is unlikely to propagate out of or spill over the edge portions of the wafer. Such a three-terminal piezoelectric filter oscillates in two modes, as indicated by curves A and B in FIG. 2, respectively called the symmetric or 110 mode and the antisymmetric or 210 mode. In the symmetric mode, the oscillating electric charge distribution is concentrated at the gap between the input and output electrodes; in the latter mode, the charge distribution is concentrated at the center portions of the input and output electrodes. The relationship between the resonant impedance of the symmetric mode Z.sub.1 and that of the antisymmetric mode Z.sub.2 is such that Z.sub.1 &lt; Z.sub.2, which results in the filter characteristic shown in FIG. 3. Such a curve departs from the optimum filter characteristic in that it has a peak ripple, has a non-uniform band pass width, and is unsymmetrical about the center frequency f.sub. o.
Attempts have been made to achieve a desired band pass width by appropriately changing the dimensions of the input, output and ground electrodes, designated V and W in FIG. 1. Such attempts have been found to vary the impedance characteristics of both of the two oscillation modes, however, and to shift the center frequency from the desired value, without substantially affecting the band pass width. Furthermore, it has been found extremely difficult to vary the band pass width while still maintaining the symmetrical distribution of each mode.